1. Field of the Invention
The present invention relates to vertical positioning of an optoelectronic component on a support that connects it optically with an optical guide integrated into said support.
2. Discussion of the Background
The development of integrated optics and the miniaturization of optoelectronic components have brought about the creation of hybrid apparatuses in which components (normally of the active type such as laser diodes, modulators, photodetectors, etc.) are mounted on a support where they can be optically connected to components described as passive (e.g. optical guides) integrated onto or into said support. Such a support may consist of a silicon substrate on which silica is deposited to produce one or more optical guides.
A considerable amount of work has been devoted to producing hybrid optoelectronic circuits. Most of this work has been on configurations in which the optical alignment between the mounted component and the optical guide may be described as simple: this is typically a photodetector lit by a beam of light conducted by an optical guide. A small proportion of this work concerns configurations in which the optical alignment between the mounted component and the optical guide constitute an optical coupling where great accuracy is required: for example, the alignment of a laser source with an optical guide. It will be understood that in this example alignment must be the best possible to give maximum efficiency of use.
A laser diode designed for mounting on a support to be connected to an integrated optical guide is a component whose transmitting strip is located near one surface of the component in order to be optically connected to an optical guide mounted on the surface of the support. When in place the laser diode is thus upside down.
Alignment is effected in all three spatial planes. When dealing with a flat support, alignment in the two planes of the support can be achieved satisfactorily using various methods known in the art (e.g. using mechanical stops, micronized shot, etc.). However, vertical alignment is difficult to achieve due to the process used to produce the optical guide. A silica on silicon optical guide (e.g. single mode 1.3 .mu.m/1.55 .mu.m) is usually produced by superimposing:
a layer of silica, known as the lower confinement layer, more than 12 .mu.m thick ensuring optical insulation with the silicon support,
a thinner guide core between 4 and 6 .mu.m thick made of silica that has been doped to increase its refractive index. This layer may be etched or not depending on whether a planar or channel-type guide is required,
a layer of silica, known as the upper confinement layer, more than 10 .mu.m thick that insulates the core from the ambient air.
The layers of silica of this type of optical guide may, for example, be deposited using PECVD (plasma-enhanced chemical vapor deposition) technology. The thickness of these layers is very difficult to determine and relative inaccuracy can be very high: typically up to 10% for the upper confinement layer, i.e. .+-.1 .mu.m, and 3% for the lower confinement layer, i.e. .+-.0.4 .mu.m. Vertical alignment is usually determined in relation to the top of the upper confinement layer, i.e. in relation to the surface of the silicon support on which the silica layers are deposited. Such mechanical references are totally incompatible with correct optical alignment of the optical guide with the mounted component (typically .+-.0.25 .mu.m for a laser diode structure).
Among the documents of the known art that concern the field of the invention, the following articles may be cited:
"Film-Level Hybrid Integration of AlGaAs Laser Diode with Glass Waveguide on Si Substrate" by M. Yanagisawa et al., published in "IEEE Photonics Technology Letters, Vol. 4, No. 1, January 1992, pages 21-23. (Reference 1),
"Silica-based Optical Waveguide on Terraced Silicon Substrate as Hybrid Integration Platform" by Y. Yamada et al., published in "Electronics Letters", Vol. 29, No. 5, pages 444-446. (Reference 2),
"A High-Density, Four-Channel OEIC Transceiver Module Utilizing Planar-Processed Optical Waveguides and Flip-Chip, Solder-Bump Technology" by K. P. Jackson et al., published in "Journal of Lightwave Technology", Vol. 12, No. 7, July 1994, pages 1185-1191. (Reference 3),
"Hybrid Integration of Semiconductor Lasers with Si-based Single-Mode Ridge Waveguides" by E. E. L. Friedrich et al., published in "Journal of Lightwave Technology", Vol. 10, No. 3, March 1992, pages 336-340, (Reference 4),
French patent FR-A-2 694 841 registered by the French Atomic Energy Commission, the equivalent of US patent U.S. Pat. No. 5,321,786. (Reference 5).
In Reference 1 the silicon support constitutes the vertical mechanical reference for mounting the component, a laser diode. The component is adjusted to the thickness of the lower confinement layer by depositing a new layer grown by epitaxis on the surface of the component designed to be positioned relative to the silicon support.
In Reference 2 the silicon support again constitutes the vertical mechanical reference for mounting the component. In this configuration the silicon lower confinement level is worked on to bring the optical guide near to the required level, i.e. close to the mechanical reference constituted by the silicon support.
In Reference 3 the top of the upper confinement layer acts as the mechanical reference; micronized shot welding techniques are also used.
In References 4 and 5, the mechanical reference is obtained by etching from the top of the upper confinement layer. In this technique two uncertainties are added: one related to the deposit and the other related to the etching.